Ore-Forming Fluid Evolution and Ore Genesis of the Cuyu Gold Deposit in Central Jilin Province, NE China: Constraints from Geology, Fluid Inclusions, and H–O–S–Pb Isotope Studies

Abstract

The Cuyu gold deposit in central Jilin Province in Northeast China is located in the eastern segment of the northern margin of the North China Craton (NCC), as well as the eastern segment of the Xing’an–Mongolian Orogenic Belt (XMOB). Gold ore-bodies are controlled by NW-trending faults and mainly occur in late Hercynian granodiorite. The mineralization process in the Cuyu deposit can be divided into three stages: quartz + coarse grained arsenopyrite + pyrite (stage I), quartz + sericite + pyrite + arsenopyrite + electrum + chalcopyrite + sphalerite (stage II), and quartz + calcite ± pyrite (stage III). Stage II is the most important for gold mineralization. We conducted analyses including petrography, microthermometry, laser Raman spectroscopy of fluid inclusions, and H–O–S–Pb isotopic analysis to elucidate the mineralization processes in the Cuyu deposit. Five types of primary fluid inclusions (FIs) are present in the hydrothermal quartz and calcite grains of the ore: liquid-rich two-phase aqueous fluid inclusions (L-type), vapor-rich two-phase aqueous fluid inclusions (V-type), CO₂-bearing two- or three-phase inclusions (C1-type), CO₂-rich two- or three-phase inclusions (C2-type), and pure CO₂ mono-phase inclusions (C3-type). From stages I to III, the fluid inclusion assemblages changed from L-, C2-, and C3-types to L-, V-, C1-, C2-, and C3-types and, finally, to L-types only. The corresponding homogenization temperatures for stages I to III were 242–326 °C, 202–298 °C, and 106–188 °C, and the salinities were 4.69–9.73, 1.63–7.30, and 1.39–3.53 wt.% NaCl equiv., respectively. The ore-forming fluid system evolved from a NaCl-H₂O-CO₂ ± CH₄ ± H₂S fluid system in stage I and II with immiscible characteristics to a homogeneous NaC-H₂O fluid system in stage III. Microthermometric data for stages I to III show a decreasing trend in homogenization temperatures and salinities. The mineral assemblages, fluid inclusions, and H–O–S–Pb isotopes indicate that the initial ore-forming fluids of stage I were exsolved from diorite porphyrite and characterized by a high temperature and low salinity. The addition of meteoric water in large quantities led to decreases in temperature and pressure, resulting in a NaCl-H₂O-CO₂ ± CH₄ ± H₂S fluid system with significant immiscibility in stage II, facilitating the deposition of gold and associated polymetallic sulfides. The Cuyu gold deposit has a similar ore genesis to those of gold deposits in the Jiapigou–Haigou gold belt (JHGB) of southeastern Jilin Province indicating potential for gold prospecting in the northwest-trending seam of the JHGB.

1. Introduction

Central Jilin Province in northeastern (NE) China, situated within the northeastern segment of the North China Craton (NCC) and adjacent to the Xing’an-Mongolian Orogenic Belt (XMOB), represents a significant polymetallic Au–Cu metallogenic zone with documented gold reserves exceeding 200 tons (Figure 1A; [1,2,3]). This region has undergone a complex tectonic evolution spanning the Late Paleozoic to Early Mesozoic, marked by the interplay between the Paleo-Asian and Paleo-Pacific oceanic tectonic regimes, including superposition and transitional dynamics [4,5,6,7,8,9,10,11,12]. Middle Jurassic magmatic intrusions facilitated the development of NW–SE-orientated hydrothermal gold systems, exemplified by the Jiapigou, Haigou, Shajingou, and Cuyu deposits. These ore-bodies are structurally governed by NW-trending fault systems, collectively forming the prominent Jiapigou–Haigou Gold Belt (JHGB). Extensive research has focused on elucidating the geological control of mineralization [13,14,15,16], temporal constraints on ore formation [17,18,19,20], and tectonic drivers of deposit genesis [21,22,23]. Based on host lithologies, regional gold deposits are categorized into three types: (1) Archean-hosted deposits (e.g., Jiapigou and Haigou [24,25,26,27,28]), (2) Paleozoic epimetamorphic rock-hosted systems within the Piaohechuan Formation (e.g., Erdaodianzi and Datudingzi; [29,30]), and (3) Mesozoic intrusion-related deposits (e.g., Cuyu and Shajingou; [31,32,33,34,35,36]). Despite recent discoveries, Mesozoic intrusion-hosted gold systems remain understudied compared to their Archean and Paleozoic counterparts, necessitating further investigations into their genetic frameworks [37,38,39,40,41,42,43].

The Cuyu gold deposit occupies the eastern periphery of the North China Craton (NCC), lying approximately 25 km south of Panshi City in central Jilin Province, northeastern China [31,32,33]. This deposit hosts documented gold reserves exceeding 10 tons, positioning it as a medium-scale resource with significant exploration potential. Previous investigations have primarily focused on its geological framework [14,22,23,32], its lithostratigraphic controls [14,22,23], and the genesis of its mineralization [14,22,23,32]. However, critical knowledge gaps persist regarding fundamental geological processes. Foremost among these is the unresolved debate surrounding the provenance of ore-forming fluids. Wang et al. (2010) propose a metamorphic fluid origin [22], contrasting Su et al. (2014), who advocate for mantle-derived magmatic fluids as the primary source [23,24]. Additionally, the temporal evolution of these fluids—specifically, their role in governing metal enrichment and precipitation mechanisms—remains inadequately characterized. Thirdly, ongoing controversies regarding the sources of ore-forming materials persist among granite [31,34], diorite porphyrite [14], and strata [14,18,26,44,45], with some researchers attributing the origin to granitic rocks, while others emphasize derivation from host stratum materials through fluid–rock interactions. The genetic classification of the Cuyu gold deposit remains contentious, with divergent interpretations including orogenic-type mineralization [23,24] and magmatic–hydrothermal vein-type models [37]. As a representative intrusion-hosted deposit within Mesozoic intermediate–acidic plutons, its genetic affinity to gold systems in the Jiapigou–Haigou Gold Belt (JHGB) remains unresolved. The clarification of this relationship could have critical implications in delineating exploration strategies in this region.

To resolve these uncertainties, this study integrated petrographic analysis, microthermometric and laser Raman spectroscopic characterization of fluid inclusions, and quartz H–O stable isotope geochemistry to determine the provenance, physicochemical attributes, and evolutionary trajectory of ore-forming fluids. Complementary S–Pb isotopic analyses of sulfides and intrusive lithologies further explored the origin of metallogenic materials. By synthesizing these findings with prior research, we established a comprehensive genetic framework for the Cuyu deposit, offering theoretical insights to guide prospecting efforts in analogous mineralization systems.

2. Geological Background

The northeastern part of China, nestled between the Siberian Craton and the NCC, constitutes the eastern portion of the Xing’an–Mongolian Orogenic Belt (XMOB; [37,38,39]). This region has been shaped by the evolution of the Paleo-Asian Ocean, the Xing’an–Mongolian orogeny, and the subduction of the Paleo-Pacific Plate [1,2,3,4,5,6,7,8,9,10,11,12,25,26]. A multitude of tectonic, magmatic, and hydrothermal activities have taken place in this region in multiple stages, resulting in the formation of numerous endogenic metal deposits. Consequently, it has become one of China’s most significant areas for polymetallic mineralization [27,28].

Central Jilin Province in northeastern China is situated along the eastern segment of the northern margin of the NCC. It is bounded to the west by the Yitong–Yilan Fault; to the east by the Dunhua–Mishan Fault; to the north by the Xar Moron–Changchun-Yanji Fault; and to the south by the Chifeng–Bayan–Kaiyuan Fault (Figure 1C). The Jinyinbie Fault, trending northwest in this area, is intersected by the northeast-trending Dunhua–Mishan Fault. This intersection is accompanied by the deposition of Mesozoic continental volcanic and sedimentary rocks, which unconformably overlie the Paleozoic rocks (Figure 1C). The region underwent a complex multi-system tectonic evolution from the Paleozoic to the Mesozoic, characterized by sequential and superimposed tectonic regimes that profoundly influenced its metallogenic architecture. During the Paleozoic, the protracted subduction of oceanic plates beneath continental margins generated a series of spatially overlapping volcanic arc systems, evidenced by calc-alkaline magmatic suites, accretionary complexes, and high-pressure metamorphic assemblages. This arc-dominated regime culminated in the Late Permian with the terminal collision of continental blocks, likely associated with the closure of the Paleo-Tethyan Ocean, which induced crustal thickening, regional metamorphism, and syn-collisional granitoid emplacement [27]. The subsequent Late Triassic transition to post-orogenic extension, driven by the lithospheric delamination or gravitational collapse of the orogenic wedge, facilitated the development of rift basins and the intrusion of A-type granites and mafic dikes, marking a shift from compressional to extensional tectonics [28]. By the Early Jurassic, the westward subduction of the Paleo-Pacific Plate beneath the continental margin re-established an active convergent setting, triggering extensive magmatic arc development, back-arc basin formation, and the accretion of subduction-related lithologies, while thermally and mechanically reworking earlier structures [29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46].

During the Middle Jurassic, the intense subduction of the Paleo-Pacific Plate triggered frequent tectonic–magmatic–hydrothermal activity, leading to the emplacement of voluminous intermediate–acidic granitoid intrusions associated with a series of large-to-super-large porphyry Mo (Cu) deposits (e.g., Daheishan, Fuanpu, and Changanpu) [32,33,45,46]. Concurrently, intermediate–mafic magmatism generated diorite and dioritic porphyry dikes, accompanied by the formation of numerous hydrothermal vein-type gold (Ag) deposits, including the Jiabigou, Haigou, Cuyu, and Shajingou deposits [18,31,35,43,44]. These gold deposits are spatially aligned along NW-trending fault zones, demonstrating structural control in regional fault systems. Collectively, these deposits form the renowned Jiapigou–Haigou Gold Belt (JHGB), with mineralization intricately tied to regional geological evolution and the subduction of the Paleo-Asian Ocean Plate.

The JHGB occupies the northeastern margin of the North China Craton (NCC), within the Longgang terrane, and borders the Xing’an–Mongolian Orogenic Belt (XMOB) to the north (Figure 1B,C). Regional stratigraphy encompasses Archean medium-to-high-grade metamorphic complexes, Proterozoic low-to-intermediate-grade metamorphic units, Paleozoic weakly metamorphosed sedimentary sequences, Mesozoic volcanic–sedimentary assemblages, and Cenozoic unconsolidated deposits (Figure 1C). Structurally, the Dunhua–Mishan Fault represents a subsidiary component of the Tancheng–Lujiang Fault Zone [37,39,40,41], while NW-striking secondary faults—including the Jiapigou, Jinyinbie, and Fuerhe systems—reflect a brittle reactivation of pre-existing ductile structures within the Dunhua–Mishan framework. The JHGB is characterized by extensive Mesozoic intrusions accompanied by a limited exposure to Early and Late Paleozoic intrusions. The widespread distribution of intrusions during the Mesozoic was driven by intense magmatic activity, which facilitated the formation of numerous gold deposits. In recent years, small-to-medium-sized gold deposits have been identified in Mesozoic intrusions on both sides of the JHGB, such as Shajingou, Songjianghe, and Cuyu. However, the factors controlling the mineralization associated with these deposits remain insufficiently studied.

3. Deposit Geology

The Cuyu gold deposit in central Jilin Province is located in the eastern segment of the northern margin of the NCC and to the northwest of the JHGB (Figure 1B; [23,24]). The main exposed strata in the ore district include the Shizui Formation of the Upper Carboniferous, the Nanloushan Formation of the Jurassic, and the Quaternary strata (Figure 2A). The Carboniferous Shizui Formation, mainly composed of andesite volcanic clastic rocks, is primarily exposed in the western part of the ore district. The Lower Jurassic Nanloushan Formation, mainly composed of dark-gray andesite, andesite porphyry, and andesite volcanic breccia, is primarily located in the northern part of the ore district. In addition, a number of intrusions are distributed in the central part of the ore district. According to cross-cutting relationships, petrography, and geochronological data, intrusions into the ore district can be categorized as late Hercynian granitoid, accompanied by some early Yanshanian diorite porphyrite veins (Figure 2A). The late Hercynian intrusions consist of syenogranite, monzogranite, and granodiorite, which are mainly distributed in the central and southern parts of the ore district. These intrusions are extensively exposed and mainly controlled by NW-trending faults. The exposed dike rocks in the ore district are mainly diorite porphyrites formed during the early Yanshan period. These diorite porphyrites exhibit a NW-trending orientation, which is attributed to the control exerted by NW-trending faults parallel to the ore-bodies (Figure 2A). Moreover, in the deep part of the ore district, diorite porphyrite veins have been discovered with obvious alterations and mineralization, running parallel to the ore-body and cutting through the granite intrusions formed during the late Hercynian.

A total of eleven lode gold ore-bodies, all hosted in altered granodiorite, have been identified in the Cuyu deposit, including two main ore-bodies and four concealed ore-bodies [22,23,24]. More than 10 tons of gold resources have been detected in the Cuyu gold deposit to date, confirming it as a medium-sized gold deposit and showing significant potential for gold prospecting in the deep part of the ore district [23]. The main ore-bearing structure is NW-trending, and the concealed ore-bodies are controlled by the NE-trending fault (Figure 2A). The main ore-bodies are concealed tens of meters below the surface. The shape of the main ore-bodies is unstable, and the thickness varies greatly, ranging from a few centimeters to several meters (Figure 2A; [23,24]). The X3 ore-body is the largest in the deposit, with a depth of 833 m and a length of 435 m along the strike direction (Figure 2B). It strikes 275–315° and dips NE at 40–70° with a gold grade of 0.13–165.40 g/t (with a mean grade of 16.88 g/t) and a silver grade ranging from 1.00 to 310.20 g/t (with a mean grade of 25.26 g/t). The X4 ore-body, which is the second largest in the ore district, mainly takes the form of veins and veinlets (Figure 2B). The ore-body is buried at a depth of 1212 m and extends 350 m along the strike direction, with a dip angle of 40–70° toward the northeast. It has an average gold grade of 16.12 g/t, with the highest gold grade of 113.69 g/t.

The ore is characterized by gold-bearing pyrite veins and polymetallic sulfide–quartz veins (Figure 3). In the Cuyu gold deposit, the ore chiefly occurs in vein structures (Figure 3A–C), with subordinate occurrences in massive structures (Figure 3D,E) and disseminated structures (Figure 3C). In ores, metal minerals primarily appear in the form of crystal (Figure 3G,H), metasomatic (Figure 3G–K), and solid solution separation textures (Figure 3G,H,K). The primary occurrence state of gold in the deposit is visible gold, predominantly found as electrum within the cracks and interstitial spaces in pyrite (Figure 3I–K). The principal metallic minerals in ores include arsenopyrite (Figure 3G,H), pyrite (Figure 3G–I), galena (Figure 3K), sphalerite (Figure 3H–J), and chalcopyrite (Figure 3G,H,J). Non-metallic minerals found in ores include K-feldspar (Figure 3L), quartz (Figure 3L–O), sericite (Figure 3M), chlorite (Figure 3N), and calcite (Figure 3O).

The hydrothermal alteration in the Cuyu deposit primarily led to K-feldspathization (Figure 3L), silicification (Figure 3M–O), sericitization (Figure 3M), chloritization (Figure 3N), and carbonatization (Figure 3O), which are extensively developed in the ore-bodies and late Hercynian diorite porphyrite veins along the NW-treading fault zone. Silicification and sericitization show the closest association with gold mineralization. Silicification is the most prevalent alteration type throughout the mineralization process (Figure 3M–O) and commonly forms vein-like structures. Sericitization is pervasive in the wall rocks near the ore-bodies, is primarily observed during the main mineralization stage, and diminishes with distance from the ore-bodies. Carbonation is a significant symbol of late mineralization. This alteration typically forms quartz–calcite veins that cut through earlier alteration zones, filling in fractures and cracks in the wall rocks.

It has been established that the formation of the Cuyu gold deposit can be divided into three stages, as evidenced by mineral assemblages, cross-cutting relationships, ore structures and textures, and associated hydrothermal alterations. These stages are (from early to late,

Table 1. Paragenetic sequence of the major minerals of the Cuyu gold deposit.

		Stage of Mineralization	Quartz-Arsenopyrite-Pyrite	Quartz-Sericite-Native Gold-Polymetallic Sulfide	Quartz-Carbonate ± Pyrite
	Sequence
Mineral

): quartz + coarse grained arsenopyrite + pyrite (stage I; Figure 3A,D), quartz + sericite + pyrite + arsenopyrite + electrum + chalcopyrite + sphalerite (stage II; Figure 3B–E), and quartz + calcite ± pyrite (stage III; Figure 3C,F).

Stage I: In this stage, the ore mineral assemblage was quartz + coarse grained arsenopyrite + pyrite (Table 1). At this stage, high-temperature hydrothermal fluids infiltrated the wall rocks, inducing K-feldspathization and resulting in the formation of coarse-grained arsenopyrite and pyrite (Figure 3L). The metallic minerals arsenopyrite and pyrite have an automorphic texture (Figure 3B,F). The subsequent cooling of the hydrothermal system promoted silicification, leading to the formation of milky quartz veins (Figure 3B).

Stage II: Stage II was the major gold mineralization stage, leading to quartz, pyrite, galena, arsenopyrite, sphalerite, chalcopyrite, and electrum (Figure 3C, Table 1). Compared with the first stage, the pyrite and arsenopyrite generated in this stage have smaller particle sizes and poorer crystal shapes. The alterations in this stage included the continuation of silicification, followed by sericitization, and chloritization (Figure 3M,N). Silicification was marked by the formation of abundant smoky gray quartz. The intrusion of potassium-rich hydrothermal fluids triggered sericitization (Figure 3M), which was associated with the deposition of polymetallic sulfide veins (pyrite + arsenopyrite + electrum + chalcopyrite + sphalerite). As the hydrothermal temperature gradually declined, mafic minerals such as pyroxene and amphibole underwent chloritization, transforming into chlorite (Figure 3N). Among these alterations, sericitization shows the strongest association with gold mineralization, and the intensity of sericitization is positively correlated with gold grade, providing evidence that this hydrothermal activity controlled mineralization in the deposit.

Stage III: This stage mainly produced non-metallic minerals, including quartz and calcite (Figure 3E,O, Table 1). Occasionally, disseminated pyrite is distributed in quartz carbonate veins (Figure 3D). During this stage, carbonatization became the dominant alteration, restricted to late-stage vein infill and accompanied by the formation of calcite. This represents the late stage of alteration and mineralization. In this stage, many quartz–calcite veins were generated that cut through early quartz sulfide veins.

4. Sample and Analytical Methods

4.1. Fluid Inclusions

The samples of quartz and calcite from the three mineralization stages were selected for studies of petrography, microthermometry and Laser Raman spectra analyses of fluid inclusions. The studies were carried out at the Laboratory of Geological Fluid of the Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Natural Resources of China, Changchun, Jilin Province, China. Fluid inclusion analysis was performed on doubly polished thin sections (0.2 mm thick-ness) of quartz and calcite. Initial optical microscopy screening identified 20 sections for combined petrographic and microthermometric analysis, with six representative samples subjected to laser Raman spectroscopy.

Microthermometric investigations employed a Linkam THMS-600 heating-freezing stage, operating across a temperature spectrum of −180 °C to +500 °C. Instrument calibration utilized certified phase transition standards: CO₂ triple-point equilibrium (−56.6 °C) and H₂O freezing-point depression (0.0 °C). Phase transition monitoring maintained a con-trolled heating rate of 0.1 °C/min to ensure measurement fidelity. Temperature measure-ment precision remained within ±0.2 °C across subzero conditions (−180 to 31 °C) and ±2 °C at elevated temperatures (>31 °C).

Gas-phase compositional analysis was conducted using an RM-2000 laser Raman microprobe equipped with a 514 nm monochromatic argon-ion laser. Spectral acquisition parameters included a scan range of 100–4300 cm⁻¹, 60 s integration time per spectrum, laser spot diameter (~1 μm), and spectral resolution of 0.14 cm⁻¹ under confocal op-tical configuration.

4.2. Hydrogen and Oxygen Isotopes

Individual quartz grains were prepared by careful handpicking under a binocular microscope to achieve a purity of 99%, and followed by cleaning in doubly distilled water. For hydrogen and oxygen isotopic analysis, quartz samples from three distinct mineralization stages of the X4 ore-body in the Cuyu gold deposit were selected and analyzed using a MAT253-EM mass spectrometer at the Beijing Research Institute of Uranium Geology’s Analytical Laboratory, part of China National Nuclear Corporation (CNNC). The determination of hydrogen isotopes involved heating the selected quartz samples in a vacuum to release H₂O from the native fluid inclusions in the mineral grains, and then reacting the extracted H₂O with Zn to produce H₂, which was finally measured for δD values on the mass spectrometer. Samples were reacted with BrF₅ and converted to CO₂ on a platinum-coated carbon rod for oxygen isotopic analysis [14,31]. The determination of oxygen isotopes was achieved by reacting the selected quartz samples with BrF₅ under vacuum (10⁻³ Pa) and at a constant temperature (500~680 °C), releasing O₂ and impurities such as SiF₄ and BrF₃. The impurities were removed by freezing, yielding pure O₂, which was then reacted with graphite at a constant temperature (700 °C) in the presence of platinum as a catalyst to produce CO₂. The CO₂ was collected by freezing and sent to the MAT253 gas isotope mass spectrometer for the determination of oxygen isotope composition. The results are reported as δ¹⁸O_V-SMOW, with an analytical precision better than ±0.2‰; the oxygen isotope standards used were GBW-04409 and GBW-04410 quartz standards, with δ¹⁸O values of 11.11 ± 0.06‰ and −1.75 ± 0.08‰, respectively.

4.3. Sulfur Isotopes

Sulfur and lead isotope compositions are one of the effective methods for determining the source of ore-forming materials [14]. Eight gold-bearing sulfide ore samples were tested for sulfur isotopes at the Beijing Research Institute of Uranium Geology’s Analytical Laboratory. Each two samples of the pyrite, chalcopyrite, and molybdenite samples were used for sulfur isotope analyses. A total of 10–100 mg of metal sulfide grains was mixed with cuprous oxide and powdered to 200 mesh for determining the sulfur isotope compositions by using a Delta v plus mass spectrometer at the Analytical Laboratory in the Beijing Research Institute of Uranium Geology, CNNC. The Sulfur isotope test analysis is to grind the pure metal sulfide single mineral sample and Cu₂O catalyst in a certain proportion (about 200 mesh) to mix it evenly, heat them to 980 °C under the condition of vacuum and pressure of 2.0 × 10⁻² Pa, generate SO₂ gas during the oxidation reaction, and collect them by freezing. The Sulfur isotope composition was determined on MAT-253-EM gas isotope mass spectrometry. The measurement results were based on CDT (denoted as δ³⁴S_V-CDT), and the analysis accuracy was better than ±0.2‰.

4.4. Lead Isotopes

In the lead isotope analysis, a sample of 0.2 g sulfide monomineral powder was weighed and placed in a dissolving tank PFA, then dissolved with a mixed acid solution (HF + HNO₃ + HClO₄). After the sample is completely dissolved, the solution is heated, and then 6 mol/L of hydrochloric acid is added to form chloride, and the solution is heated again. 1 mL 0.5 mol/L HBr solution was dissolved and centrifugation was started. An anion exchange column (250 μL, AG1 × 8, 100–200 mesh) was added to the supernatant, the impurities were washed with HBr solution with a concentration of 0.5 mol/L, and 1 mL (6 mol/L) of HCl solution was used as lead in a polytetrafluoroethylene beeper, then dried. Lead isotope analyses were conducted on 20 sulfide and intrusion-hosted samples at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology. Measurements were performed using an IsoProbe-T thermal ionization mass spectrometer (TIMS), with samples loaded onto rhenium filaments using a phosphoric acid-silica gel emitter. Isotopic ratios were determined in static multicollection mode, achieving an analytical precision better than ±0.09‰ (2σ) for Pb isotope determinations.

5. Analytical Results

5.1. Fluid Inclusions Characteristics

5.1.1. Petrography and Types of Fluid Inclusion

Numerous primary fluid inclusions formed during the three mineralization stages of the Cuyu deposit (Figure 4). In the Cuyu gold deposit, fluid inclusions are spatially associated with growth zones of quartz grains or occur as isolated features within these zones. Based on detailed petrographic observations, we therefore interpret that all analyzed fluid inclusions in the Cuyu deposit are primary in origin, following the criteria established by Roedder (1984) for distinguishing primary versus secondary inclusions [47]. Three primary fluid inclusion types—liquid-rich aqueous (L-type), vapor-rich aqueous (V-type), and CO₂-bearing (C-type)—were identified in the Cuyu gold deposit through integrated analysis of phase behavior at ambient conditions (25 °C), phase transitions during thermal cycling (cooling to heating), and laser Raman spectroscopy. Their diagnostic characteristics are detailed below.

(1)

L-type: liquid-rich aqueous fluid inclusions

The L-type fluid inclusions are abundant in quartz and calcite grains from stage I to stage III, consisting of a liquid (L_H2O) and a vapor (V_H2O)water phase with V_H2O/(V_H2O + L_H2O) < 50 vol% (mainly 5–40 vol%) at room temperature (25 °C, Figure 4A–G). The L-type fluid inclusions usually have euhedral, oval or irregular shapes, and are 3–22 μm in size. This type of fluid inclusion is completely homogenize to an aqueous solution phase during heating. They occur mainly in isolation, as well as coexist with V-type and C-type fluid inclusions in the same quartz.

(2)

V-type: vapor-rich aqueous fluid inclusions

This type of fluid inclusions mainly developed in quartz in stage II, composed of liquid (L_H2O) and a vapor (V_H2O) water phase with V_H2O/(V_H2O + L_H2O) > 50 vol% (mainly 80–95 vol%) at room temperature (25 °C, Figure 4D,F,G). V-type fluid inclusions have subcircular, oval and irregular shapes, with a particle size of 4–25 μm. They are mainly intergrowth with L-type and C-type in quartz (Figure 4D–G). During heating, this type of fluid inclusion completely homogenizes to a vapor phase.

(3)

C-type: CO₂ fluid inclusions

This type of fluid inclusion in the Cuyu gold deposit can be observed in two or three phases at room temperature (25 °C), mainly composed of different phase of CO₂ and aqueous solution or entirely of CO₂. According to the proportion of CO₂ phases and completely homogeneous results, these fluid inclusions can be subdivided into three subtypes: CO₂-bearing fluid inclusions (C1-type), CO₂-rich fluid inclusions (C2-type) and CO₂-dominated inclusions (C3-type). They are 3–25 μm in diameter and are strip or elliptical in shape. The C1-type and C2-type fluid inclusions are mainly composed of three (L_CO2 + V_CO2 + L_H2O) or two (L_H2O + L_CO2) phases at room temperature. Notably, the fluid inclusion composed of two phases changes into three phases with the appearance of CO₂ bubbles that freeze at 10 °C.

C1-Type: CO₂-Bearing Fluid Inclusions

The C1-type fluid inclusions have oval or strip in shape, with 5–20 μm. This type fluid inclusion occurs in the stage II, coexisting with L-type, V-type, C2-type and C3-type fluid inclusions (Figure 4G). This distinctive coexistence suggests that during the evolution of hydrothermal fluids, abrupt pressure release or the influx of meteoric water triggered vigorous phase separation events, resulting in the instantaneous dissociation of initially homogeneous hydrothermal fluids into immiscible fluid systems [31,38]. C1-type fluid inclusions have the CO₂ volumetric proportions [(V_CO2 + L_CO2)/(V_CO2 + L_CO2 + L_H2O)] < 50 vol% (mainly 20–40 vol%) at room temperature and homogenize to an aqueous solution phase during heating.

C2-Type: CO₂-Rich Fluid Inclusions

This type of fluid inclusion mainly developed from stage I to stage II and is randomly distributed as an oval or strip shapes, with a particle size of 4–24 μm (Figure 4B,D–K). C2-type fluid inclusions have CO₂ volumetric proportions [(V_CO2 + L_CO2)/(L_H2O + L_CO2 + V_CO2)] > 50 vol% (mainly 75–95 vol%) and homogenize to the liquid CO2 phase during heating.

C3-Type: CO₂-Pure Fluid Inclusions

C3-type fluid inclusions composed of pure-liquid CO₂ phase is mainly abundant at stage I and stage II and randomly distributed as an elliptical shape, with 3–24 μm (Figure 4C,H). This type of fluid inclusions changes into two phase (L_CO2 + V_CO2) with the appearance of CO₂ bubbles that freeze at ~10 °C.

5.1.2. Microthermometry

The results of microthermomentry of primary fluid inclusions in quartz and calcite from three mineralization stages of the Cuyu gold deposit are listed in

Table 2. Microthermometric data and calculated parameters for fluid inclusions in the Cuyu gold deposit.

Mineralized Stages	Host Minerals	Inclusion Types	Tm-CO2 (°C)	Tm-cla (°C)	Th-CO2 (°C)	Tm-ice (°C)	Concentrating T (°C)	Salinity (wt.% NaCl eq)
stage I	Qz	L-type	/	/	/	−6.4 to −3.4	242–326	5.50–9.73
		C2-type	−58.3 to −57.9	5.5–6.7	26.7–29.2	/	265–323	4.69–8.29
		C3-type	−58.4 to −57.6	/	26.4–29.5	/	/	/
Stage II	Qz	L-type	/	/	/	−4.6 to −2.8	202–293	4.63–7.30
		V-type	/	/	/	−3.9 to −1.6	211–291	2.73–6.29
		C1-type	−59.2 to −58.1	7.3–8.8	26.6–29.8	/	242–298	2.42–5.23
		C2-type	−59.4 to −58.5	7.6–9.2	25.9–30.1	/	249–284	1.63–4.69
		C3-type	−59.0 to −58.2	/	26.9–28.9	/	/	/
Stage IV	Qz, Cc	L-type	/	/	/	−2.1 to −0.8	106–188	1.39–3.53

Tm-CO₂ = final melting temperature of solid CO₂; Tm-cla = final melting temperature of CO₂–H₂O clathrate; Th-CO₂ = homogenization temperature of the CO₂ phases; Th-total = total homogenization temperature of CO₂ fluid inclusion; Tm-ice = temperature of final ice melting; Qz = quartz; Cc = calcite.

and Figure 5. In the NaCl-H₂O system, the salinity of L-type fluid inclusions is determined by ice melting temperatures (T_m-ice), with salinities expressed as weight percent equivalent NaCl. In the H₂O-NaCl-CO₂ system, CO₂ clathrate melting temperatures (T_m-cla) are used to determine the salinity of C-type fluid inclusions.

Quartz-arsenopyrite-pyrite (stage I): The quartz in stage I mainly contains L-type, C2-type and C3-type fluid inclusions (Figure 4A–C). During heating, the L-type fluid inclusions homogenized to a single phase aqueous solution with homogenization temperatures of 242–326 °C (mainly concentrated in 280–300 °C, Figure 5A). The final ice-melting temperatures (T_m-ice) range from −6.4 to −3.4 °C, with calculated salinities range from 5.55 to 9.37 wt.% NaCl equiv. (Figure 5B). The C2-type fluid inclusions exhibit first melting temperatures of solid CO₂ (Tm-CO₂) ranging from −58.3 to −57.9 °C. These values, falling below the triple point of pure CO₂ (−56.6 °C), indicate the presence of volatile impurities (e.g., CH₄, H₂S, N₂) that depress the CO₂ phase transition temperature. The CO₂ phase homogenized to the liquid CO₂ phase at temperatures (T_h-CO2) between 26.7 and 29.2 °C, and all C2-type fluid inclusions homogenized to the liquid CO₂ phase at temperatures ranging from 265 to 323 °C (peaking at 280–300 °C, Figure 5A). In the C2-type fluid inclusions, clathrate melting temperatures (T_m-cla) are between 5.5 and 6.7 °C, calculating salinities range from 4.69 to 8.29 wt.% NaCl equiv. (Figure 5B). The initial melting temperatures of solid CO₂ of C3-type fluid inclusions range from −58.4 to −57.6 °C, which is notably lower than the triple-phase temperature of pure-CO₂ at −56.6 °C. This discrepancy implies the possible presence of trace gaseous impurities, such as CH₄, H₂S and N₂ [33,35,36]. The CO₂ phase homogenized to a liquid CO₂ phase at temperatures of 26.4–29.5 °C.

Quartz-sericite-native gold-polymetallic sulfide (stage II): The quartz in this stage contains all type of fluid inclusions, including L-type, V-type and C-type (Figure 4D–K). The L-type fluid inclusions homogenized to the aqueous solution phase at temperatures range from 202 to 293 °C, peaking at 240–260 °C (Figure 5C). The L-type fluid inclusions final ice-melting temperatures (T_m-ice) vary from −4.6 to −2.8 °C and calculate salinities of 4.63–7.30 wt.% NaCl equiv. (Figure 5D). The V-type fluid inclusions homogenized to the vapor phase at temperatures varying from 211 to 291 °C, mainly at 240–260 °C. By the way, the V-type fluid inclusions yield final ice melting temperatures (T_m-ice) at between −3.9 and −1.6 °C with salinities of 2.73–6.29 wt.% NaCl equiv. The C-type fluid inclusions which include C1-type, C2-type and C3-type fluid inclusions, with the first melting temperatures of solid CO₂ (T_m-CO2) ranging from −59.4 to −58.1 °C, are slightly lower than the triple-phase point for CO₂-pure fluid inclusions (−56.6 °C), and indicate the presence of a minor amount of CH₄, H₂S and N₂ [33,35,36]. The C1-type fluid inclusions are totally homogenized to the aqueous solution phase at temperatures varying from 242 to 356 °C, mainly at 260–280 °C. The solid CO₂ phase homogenized to a liquid CO₂ phase at temperatures (T_h-CO2) between 26.6 and 29.8 °C (Figure 5C). The clathrate melting temperatures (T_m-cla) range from 7.3 to 8.8 °C, corresponding to salinities of 2.42–5.23 wt.% NaCl equiv. (Figure 5D). The partial homogenization temperatures of C2-type fluid inclusions vary from 25.9 to 30.1 °C, and completely homogenized to liquid phase CO₂ with the complete homogenization temperatures range from 249 to 284 °C, mainly aggregation in 260~280 °C (Figure 5C). The clathrate melting temperatures (T_m-cla) are between 7.6 and 9.2 °C, corresponding to salinities of 1.63–4.69 wt% NaCl equiv. (Figure 5D). In the C3-type fluid inclusions, the phase of liquid and vapor CO₂ phase are homogenized to liquid CO₂ phase, and the homogenization temperature is 26.9–28.9 °C

Quartz-carbonate ± pyrite (stage III): Only the L-type fluid inclusions were identified in quartz and calcite grains. They are completely homogenized to the aqueous solution phase at temperatures between 106 and 188 °C, mainly between 144 and 166 °C (Figure 5E). The final ice-melting temperatures (Tm-ice) range from −2.1 to −0.8 °C, corresponding to salinities of 1.39–3.53 wt% NaCl equiv. (Figure 5F)

5.1.3. Laser Raman Spectroscopy

Laser Raman spectroscopic analysis of representative fluid inclusions across mineralization stages in the Cuyu deposit reveals distinct vapor-phase compositional trends (Figure 6). During Stage I (quartz-arsenopyrite-pyrite), L-type inclusions exhibit vapor phases dominated by CO₂ and H₂O, with trace CH₄ (Figure 6A). Concurrently, C2-type inclusions in this stage contain predominantly CO₂ with subordinate H₂S (Figure 6B). In Stage II (quartz-sericite-native gold-polymetallic sulfide), V-type inclusions display vapor compositions of H₂O and CO₂ accompanied by minor H₂S (Figure 6C), while C1- and C3-type inclusions are characterized by CO₂-dominated vapors with trace CH₄, H₂S, and N₂ (Figure 6D,E). By Stage III (quartz-carbonate ± pyrite), L-type inclusions lack detectable CO₂, transitioning to a purely aqueous vapor phase (Figure 6F). The above results show that the ore-forming fluid in the early stage and the main mineralization stage (stage I and II) of the Cuyu gold deposit was the NaCl-H₂O-CO₂ ± CH₄ ± H₂S system, and in the post-mineralization stage (stage III), the ore-forming fluid evolved into the NaCl-H₂O system.

5.2. H-O Isotopes

The data of H-O isotopes in quartz samples at different mineralization stage of the Cuyu gold deposit are shown in

Table 3. Hydrogen and oxygen isotopic data for fluids in quartz from the Middle Jurassic gold deposits in the eastern section of the northern margin of the NCC.

Ore Deposits	Sample No.	Stage	Mineral	δ18OV-SMOW (‰)	T (°C)	δ18OH2O (‰)	δD (‰)	Reference
Cuyu	8CY-1	Early stage of mineralization	Quartz	11.4	292	4.21	−83.9	This paper
	8CY-2			10.4	292	3.21	−82.2
	8CY-3	Main stage of mineralization		11.7	252	2.84	−90.5
	8CY-4			11.1	252	2.24	−90.9
	8CY-5			9.5	252	0.64	−89.9
	7CY-5			11.2	252	2.34	−91.8
	7CY-6			11.5	252	2.64	−91.1
	7CY-3	Late stage of mineralization		11.4	150	−4.09	−92.2
	7CY-7			11.4	150	−4.09	−95.4
Erdaogou		Early stage of mineralization	Quartz	9.1	330	3.2	−92.4	[39]
				11.9	330	6	−88.9
				11.8	320	5.6	−92	[40]
				10.6	315	4.2	−81
		Main stage of mineralization		11.9	230	1.9	−95	[41]
				11.2	230	1.2	−94
				14.1	230	4.1	−98
				11.2	200	−0.5	−94	[42]
		Late stage of mineralization		11.9	150	−4.6	−95
Haigou		Early stage of mineralization	Quartz	12.6	370	6.7	−80	[18]
				12.7	370	6.8	−80
				13.2	370	8.4	−74
		Main stage of mineralization		12	320	7	−98
				12	310	7.5	−105
Xiaobeigou		Early stage of mineralization	Quartz	11.6	320	5.5	−97	[42]
				4	425	0.5	−92	[43]
				7.9	310	1.4	−92	[39]
				6.6	310	0.1	−88
				8	315	1.6	−96
		Main stage of mineralization		11.6	200	−0.1	−97	[40]
				8.3	240	−1.1	−102	[18]
				7.6	240	−1.8	−102
				9.1	190	−3.3	−92	[41]
				11.3	190	−1.1	−90
		Late stage of mineralization		4.8	165	−9.4	−78	[18]
				4.4	165	−9.2	−78
				12.8	150	−2.7	−97	[41]
				9.1	167	−5	−92	[40]
				11.3	167	−2.8	−90
Bajiazi	87C743	Early stage of mineralization	Quartz	/	305	4.4	−93.9	[49]
	87C770			/	305	4.4	−89.1
Sandaogou	87C827	Early stage of mineralization	Quartz	/	345	5	−88.2	[49]
	3-1					4.2	−70	[43]
Shajingou	17SJG-1	Early stage of mineralization	Quartz	10.8	360	5.8	−83.7	[31]
	17SJG-2			11	360	6	−84.2
	17SJG-3	Main stage of mineralization		8.6	314	2.2	−95.4
	17SJG-4			8	314	1.6	−94.2
	17SJG-5	Late stage of mineralization		2.8	230	−7.1	−89.6
	17SJG-6			3.1	230	−6.8	−90.7

T = average homogenization temperatures of fluid inclusions.

. The δ¹⁸O_H2O values of the Cuyu gold deposit were calculated using the oxygen isotope fractionation equation for the quartz-water system proposed by Zheng (1993) [48]:

$${\delta }^{18}{\mathrm{O}}_{\mathrm{H}2\mathrm{O}}={\delta }^{18}{\mathrm{O}}_{\mathrm{q}\mathrm{u}\mathrm{a}\mathrm{r}\mathrm{t}\mathrm{z}}-3.38({10}^6/T^{-2})+3.4$$

The δ¹⁸O_H2O values of stage I quartz range from 3.21% to 4.21%, and the δD_V-SMOW values vary from −83.9% to −82.2%. The δ¹⁸O_H2O values of main stage of mineralization (stage II) and late stage of mineralization (stage III) are 0.64%–2.84% and −4.09%, respectively. The δD_V-SMOW values range from −91.8‰ to −89.9‰ at stage II and vary from −95.4‰ to −92.2‰ at stage III.

5.3. S Isotopes

Table 4. Sulfur isotope data of sulfide minerals from the Middle Jurassic gold deposits in the eastern section of the northern margin of the NCC.

Ore Deposits	Sample No.	Ore	Analytical Mineral	δ34S (‰)	Reference
Cuyu	7CY-3-1	Gold-bearing sulfide ore	Galena	−3.2	This paper
	7CY-3-1		Arsenopyrite	−3
	7CY-3-1		Pyrite	−5.6
	7CY-3-2		Arsenopyrite	−4.2
	7CY-3-2		Pyrite	−6.2
	7CY-3-2		Galena	−6.5
	7CY-3-3		Pyrite	−2.8
	7CY-3-4		Pyrite	−0.9
Jiapigou	BQ0835-1	Gold-bearing sulfide ore	Pyrite	8.1	[40]
	LS6-1		Pyrite	7.8
	LS6-1		Chalcopyrite	7.6
	XT200-1		Pyrite	8.8
	XT200-1		Galena	3.7
	XT200-4		Pyrite	8.4
	XT200-4		Chalcopyrite	7.3
	XT200-4		Galena	5.4
Sandaocha	Sd740119-2	Gold-bearing sulfide ore	Pyrite	6.1
	SD150118-1		Pyrite	6.1
	SD150118-1		Chalcopyrite	5.6
	SD150126-1		Pyrite	10.9
	290-162		Pyrite	5.7	[39]
	290-142		Pyrite	5.4
	J21		Pyrite	5.2
	J23		Pyrite	5.3
	J24		Pyrite	6.3
	J17		Pyrite	6
	J18		Pyrite	6
	J20		Pyrite	5.5
	380-102		Pyrite	7
	335-8		Pyrite	5.5
	335-12		Pyrite	6.4
	335-9		Pyrite	6.8
	380-7		Pyrite	7.1
	380-118		Pyrite	5.9
Erdaogou	ED1050128	Gold-bearing sulfide ore	Pyrite	6.3	[40]
	ED1050128		Chalcopyrite	5.7
	ED1050128		Galena	3.6
	ED1150119-1		Pyrite	7.4
	ED1140105-4		Pyrite	7.4
	ED1090		Pyrite	5.5
	ED1090		Chalcopyrite	5.7
	ED1090		Galena	4.1
	87C794		Pyrite	6.2	[39]
	87C794		Galena	4.2
Xiaobeigou	BG890164	Gold-bearing sulfide ore	Pyrite	9.5	[40]
	BG890142		Pyrite	7.9
	BG650181-2		Pyrite	8.8
	BG890102-1		Pyrite	9.8
Banmiaozi	100-9	Gold-bearing sulfide ore	Chalcopyrite	9.9	[15]
	160-2		Pyrite	2.7
	180-9		Pyrite	7.9
	140-3		Pyrite	9.9
	6-17		Pyrite	9.9
	190-1		Pyrite	5.8
Bajiazi	BJZ280373-1	Gold-bearing sulfide ore	Pyrite	7.2	[40]
	BJZ560413-7		Pyrite	7.9
			Pyrite	7	[39]
			Pyrite	7.9
	87C743		Pyrite	6.8
	87C743		Pyrite	−0.2

lists the S isotope compositions of eight sulfide samples from the Cuyu deposit. The δ³⁴S isotopes values of the sulfide samples range from −6.5‰ to −0.9‰, having an average value of −4.0‰ (Table 4).

5.4. Lead Isotopes

The lead isotopes data of metal-sulfide ores, ore-bearing syenite granite and ore-bearing diorite porphyrite from the Cuyu gold deposit are listed in

Table 5. Lead isotope ratios of sulfide minerals and associated intrusions in the Cuyu gold deposit.

Sample No.	Sample Description	Analytical Object	208Pb/204Pb	Error	207Pb/204Pb	Error	206Pb/204Pb	Error	Reference
7CY-3-1	Ore	Galena	38.393	0.005	15.585	0.002	18.649	0.002	This paper
7CY-3-1		Arsenopyrite	38.313	0.004	15.558	0.002	18.628	0.002
7CY-3-1		Pyrite	38.328	0.005	15.567	0.002	18.632	0.003
7CY-3-2		Arsenopyrite	38.424	0.007	15.603	0.001	18.659	0.001
7CY-3-2		Pyrite	38.335	0.004	15.567	0.001	18.634	0.002
7CY-3-2		Galena	38.335	0.004	15.57	0.002	18.629	0.002
7CY-3-3		Pyrite	38.462	0.004	15.606	0.001	18.659	0.002
7CY-3-4		Pyrite	38.276	0.003	15.56	0.001	18.565	0.002
7CY-1-1	SG	Whole rock	38.643	0.004	15.594	0.002	18.974	0.002
7CY-1-2		Whole rock	38.823	0.011	15.641	0.004	19.059	0.005
7CY-1-3		Whole rock	38.711	0.005	15.594	0.002	19.012	0.003
7CY-1-4		Whole rock	38.65	0.004	15.599	0.002	18.975	0.002
7CY-2-1		Whole rock	38.746	0.003	15.605	0.001	18.903	0.002
7CY-2-2		Whole rock	39.196	0.01	15.708	0.003	19.179	0.003
7CY-2-3		Whole rock	38.719	0.005	15.603	0.002	18.971	0.002
7CY-2-4		Whole rock	38.561	0.005	15.594	0.002	18.883	0.003
8CY-1-1	DP	Whole rock	38.332	0.004	15.574	0.002	18.621	0.0002
8CY-1-2		Whole rock	38.329	0.004	15.569	0.001	18.594	0.0002
8CY-1-3		Whole rock	38.255	0.004	15.56	0.002	18.547	0.0003
8CY-1-4		Whole rock	38.361	0.004	15.582	0.002	18.638	0.0002

SG = Syenitegranite; DP = Diorite porphyry.

. Analyses of the lead isotopes in metal-sulfide ores reveal a variation in the ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios, specifically from 18.565 to 18.659, 15.558 to 15.606, and 38.276 to 38.462, respectively. The lead isotope ratios of ore-bearing syenite granite are as follows: ²⁰⁶Pb/²⁰⁴Pb ranges from 18.883 to 19.179, ²⁰⁷Pb/²⁰⁴Pb from 15.594 to 15.708, and ²⁰⁸Pb/²⁰⁴Pb from 38.561 to 39.196. Ore-bearing diorite porphyrite exhibit range in their lead isotope ratios: ²⁰⁶Pb/²⁰⁴Pb (18.547–18.638), ²⁰⁷Pb/²⁰⁴Pb (15.560–15.582), and ²⁰⁸Pb/²⁰⁴Pb (38.255–38.361).

6. Discussion

6.1. Origin of Ore-Forming Fluids and Material

During the ascent of hydrothermal fluids in ore-forming systems, particularly in the main and late mineralization stages, a commonly observed phenomenon involves a significant influx of meteoric water into the ascending fluids. Consequently, stable isotope compositions (δ¹⁸O_H2O and δD) derived from early-stage hydrothermal minerals are critical in tracing the original source and physicochemical characteristics of primary ore-forming fluids, as later stages are often overprinted by fluid mixing [50,51]. In the Cuyu gold deposit, early-stage (stage I) H-O isotope data indicate that the fluids were not purely magmatic or metamorphic in composition; however, their isotopic signatures closely resemble those of magmatic fluids. We calculated the δ¹⁸O_H2O values for early-stage hydrothermal quartz fluid inclusions as ranging from +3.21‰ to +4.21‰ (mean = +3.71‰) and we plotted the corresponding δD values outside of the fields of magmatic and metamorphic water on a δD versus δ¹⁸O_H2O diagram (Figure 7). However, the data plot is near the magmatic water field, and it is likely that the ore-forming fluids are magmatic in origin. This interpretation is reinforced by two key lines of evidence: Firstly, the δ¹⁸O_V-SMOW values exhibit a restricted range from 10.4‰ to 11.4‰ (mean = 10.9‰), which contrasts with the significantly higher δ¹⁸O signatures (>13‰) characteristic of metamorphic fluid-dominated gold systems [52]. Secondly, the early-stage fluid inclusions yield δD values ranging from −83.9‰ to −82.2‰, distinctly more negative than orogenic gold deposits (−65‰ to −35‰; [53]). These isotopic signatures, combined with the systematic spatial association with Mesozoic diorite porphyrite veins, demonstrate that the ore-forming fluids were principally derived from magmas rather than metamorphic devolatilization processes. In contrast, stage II and III fluids display progressively depleted δ¹⁸O_H2O values of 0.64–2.84‰ (mean = 2.1‰) and −4.09‰, respectively, accompanied by δD shifts toward the Meteoric Water Line (Figure 7). This systematic isotopic evolution reflects increasing contributions from meteoric water during the later mineralization stages. The pronounced negative δ¹⁸O_H2O excursion in stage III (−4.09‰) exceeds typical magmatic fractionation trends, unequivocally indicating large-scale mixing between ascending magmatic fluids and infiltrating meteoric water. Based on the above analysis, we concluded that the ore-forming fluids of the Cuyu gold deposit were primarily derived from magmatic sources, with a progressive influx of meteoric water during the hydrothermal fluid ascent.

A summary of the δD–δ¹⁸O_H2O values of representative gold deposits in the JHGB is presented in Figure 7 and Table 3. Notably, the early-stage isotopic signatures from the Cuyu gold deposit exhibit remarkable consistency with those of other JHGB deposits, strongly suggesting a genetic link between magmatic activity and mineralization processes.

In the JHGB gold deposits, the isotopic data reveal a distinct trend of the ore-forming fluids transitioning from near-magmatic water to meteoric water. The initial fluids were predominantly derived from magmatic sources. However, during stages II and III, there was a progressive increase in the mixing of meteoric waters with magmatic fluids.

This fluid evolution trajectory was characterized by the increasing participation of meteoric water as the hydrothermal fluids ascended. This pattern indicates that the fluids underwent significant changes in their composition and origin throughout the mineralization process. Initially, the fluids were closely associated with the magmatic system, reflecting the high temperatures and pressures of the deep crustal environment. As the fluids rose towards the surface, they encountered cooler conditions and interacted with the surrounding rocks, leading to the incorporation of meteoric water components.

Sulfur isotope geochemistry serves as a critical tracer for deciphering metal and sulfur sources in ore-forming systems [55,56,57]. However, the sulfur isotopic signature (δ³⁴S) preserved in sulfide minerals is subject to modification by multiple physicochemical parameters during mineralization, including the temperature, pH conditions, redox state, and oxygen fugacity [58,59]. Hydrothermal systems exhibit a complex δ³⁴SΣS signature, which includes the sulfur isotopic compositions of sulfides (e.g., pyrite, chalcopyrite, sphalerite, galena) and sulfates (e.g., barite, gypsum) [60]. According to previous research, when the escape of ore-forming fluid oxygen is minimal, sulfur within the fluid is present in the form of HS⁻ and S²⁻, leading to pyrite precipitation with δ³⁴S values that mirror the fluid’s composition; the average δ³⁴S value can be considered indicative of the total sulfur value of the ore-forming fluid [61,62]. Under high oxygen fugacity conditions, sulfur in the fluid precipitates as SO₄²⁻, leading to the formation of sulfate minerals enriched in ³⁴S. Consequently, the sulfur isotopic composition of the precipitated pyrite is lower than that of the δ³⁴S value of the fluid system. The Cuyu gold deposit is characterized by metallic sulfides as the primary sulfide-bearing minerals, with no significant development of sulfate minerals, indicating a reduced environment for ore formation.

The δ³⁴S_V-CDT values of the Cuyu gold deposit vary from −6.5 to −0.9‰ (average −4.1‰; Figure 8 and Table 4). Although the δ³⁴S isotope values of the Cuyu gold deposit appear slightly negative, clustering near 0‰ (ranging from −6.5 to −0.9‰), this is consistent with the canonical range characteristic of magmatic–hydrothermal systems (−10 to +10‰; Figure 8a). This indicates a magmatic origin, but it may be mixed with crustal material [61,62,63,64]. According to the values for sulfide minerals from the Middle Jurassic gold deposits in the eastern section of the northern margin of the NCC, although the Cuyu values are lower, they still fall within the same isotopic regime and may reflect variation in sulfur source contributions or fluid evolution, such as Jiapigou (δ³⁴S 3.7–8.8‰, average 7.1‰), Sandaocha (δ³⁴S 5.2–10.9‰, average 6.3‰), Erdaogou (δ³⁴S 4.1–7.4‰, average 5.6‰), Xiaobeigou (δ³⁴S 7.9–9.8‰, average 9.0‰), Daxiangou (δ³⁴S 5.3–9.4‰, average 7.7‰), Banmiaozi (δ³⁴S 2.7–9.9‰, average 7.7‰), or Bajiazi (δ³⁴S −0.2 to 7.9‰, average 6.1‰; Table 4 and Figure 8b).

Based on the data analysis above, the δ³⁴S isotope values of these deposits are also consistent with the canonical range characteristic of magmatic–hydrothermal systems (−10 to +10‰). These indicate that the ore-forming material source of the Cuyu gold deposit is the same as that of typical gold deposits in the JHGB, i.e., of magmatic–hydrothermal origin with a mixture of crustal material.

Lead isotopes can provide information on crustal evolution and metal sources, primarily due to the low concentration of uranium and thorium in the sulfides [65]. In the ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagrams (Figure 9), pyrite from the Cuyu gold deposit exhibits Pb isotopic ratios predominantly plotted between the mantle and orogenic belt fields, while showing close correspondence with the isotopic composition of diorite porphyry. Considering this along with Pb isotopic evidence from the Cuyu gold deposit, we propose that the ore-forming materials were principally derived from Middle Jurassic diorite porphyry magmatism. Furthermore, the results are also consistent with isotopic chronology data. The ore-forming age of the ore-body (169.3 ± 2.0 Ma; unpublished data) is closely correlated with that of the diorite porphyrite (170.4 ± 1.8 Ma; unpublished data) but sharply contrasts with the zircon U-Pb age of syenite–granite (254 ± 2 Ma; [22,23]). These characteristics highlight the significant contribution of mantle-derived materials to the ore-forming fluids, as well as local crustal contamination during fluid–rock interactions [66]. Our results indicate that Pb was derived from the mantle, with the notable involvement of local crustal components during the fluid–rock interaction process. In conclusion, we propose that the ore-forming fluids and materials of the Cuyu gold deposit originated from the diorite porphyrite veins.

6.2. Nature and Evolution of Ore-Forming Fluids

Stage I quartz grains predominantly host coexisting L-, C2-, and C3-type fluid inclusions, which exhibit irregular spatial distributions. Microthermometric analyses reveal comparable total homogenization temperatures for L- and C2-type inclusions, indicating near-synchronous entrapment [38]. Fluid immiscibility during this stage is evidenced by the coexistence of diverse inclusion types within individual quartz grains, reflecting disequilibrium trapping conditions [47]. This interpretation is further supported by contrasting phase behaviors: L-type inclusions homogenize to aqueous phases, while C2-type counterparts transition to liquid CO₂, consistent with phase separation in a heterogeneous fluid system [31,67,68,69,70]. The sporadic presence of CO₂-rich C3-type inclusions further corroborates immiscibility [31,71,72]. Considering H-O isotopic signatures and laser Raman spectroscopic data, the stage I ore-forming fluids were interpreted as a medium–high-temperature (240–300 °C), low-salinity NaCl-H₂O-CO₂ system, primarily sourced from magmatic–hydrothermal reservoirs with minor meteoric water contributions.

Au and associated metal sulfides were mainly deposited in stage II, and the fluid inclusions in this stage are characterized by V-type fluid inclusions and C1-type fluid inclusions. Coexisting fluid inclusions have been identified in the Cuyu gold deposit and are commonly observed in individual quartz grains in stage II (Figure 4D–K). Fluid inclusions in stage II have a similar total homogenization temperature (Figure 5C). Different types of fluid inclusion have different homogeneous forms, which indicates that the fluid in stage II had obvious characteristics of fluid immiscibility, which is the main factor leading to the accumulation and precipitation of natural gold and a high number of metal sulfides [31,73]. Based on the H-O isotopes, the trapping temperatures and salinities of the fluid inclusions show a decreasing trend from stage I to stage II. Furthermore, the δD and δ¹⁸O values of stage II are lower than those of stage I and fall between meteoric and magmatic water in the δD–δ¹⁸O diagram. These indicate that the ore-forming fluids in stage II partially mixed with meteoric water, leading to a decrease in fluid temperature, which resulted in fluid immiscibility. The fluid immiscibility mechanism, in turn, resulted in the exsolution of volatiles (CO₂ and H₂S; [74]). The ore-forming fluid in stage II is a NaCl-H₂O-CO₂ ± CH₄ ± H₂S system, exhibiting mesothermal, low-salinity, and immiscibility characteristics.

Late-stage quartz exclusively contains L-type inclusions with markedly reduced homogenization temperatures and salinities (Table 2, Figure 4L,M). Laser Raman spectroscopy identified a pure H₂O composition (Figure 6F), suggesting volatile loss (CO₂, H₂S, CH₄) through fluid immiscibility during prior mineralization stages. Corresponding isotopic signatures (δD = −92.2‰ to −95.4‰; δ¹⁸O_H2O = −4.09‰) exhibit stronger affinity to meteoric water (Figure 7), confirming an evolution into a low-temperature, low-salinity NaCl-H₂O system through sustained meteoric influx.

In summary, magmatically derived NaCl-H₂O-CO₂ fluids in stage I with medium–high temperatures and low salinity circulated through the system, triggering the initial pre-enrichment of gold. Stage II witnessed significant fluid evolution as increasing meteoric water influx caused progressive temperature reduction and phase separation. This immiscibility in the NaCl-H₂O-CO₂ ± CH₄ ± H₂S system effectively destabilized gold complexes, leading to extensive sulfide precipitation and Au deposition. The final stage, stage III, showed complete fluid system transformation and evolution into a low-temperature, low-salinity, and homogeneous NaCl-H₂O system in which ore-forming fluids converged toward the meteoric water line.

Fluid immiscibility has been recognized as a critical gold precipitation mechanism in the JHGB (e.g., Jiapigou, Erdaogou, Xiaobeigou; [18,75,76,77]). Three lines of evidence substantiate immiscibility processes during the main mineralization stage of the Cuyu gold deposit: (1) Cluster distributions of C2-type, L-type, and minor C3-type inclusions within individual quartz grains demonstrate synchronous entrapment [38]. (2) C2-type inclusions are homogenized to the liquid CO₂ phase, C1-type inclusions are homogenized to the liquid phase, and L-type inclusions with different gas–liquid ratios have similar homogenization temperatures, reflecting a non-uniform state during fluid capture [69,78]. (3) Systematic Th differences between C2-type and coeval L-type inclusions (Figure 5) conform to immiscibility criteria [68]. Given that immiscibility processes preclude pressure correction requirements [79], homogenization temperatures directly reflect trapping conditions [80,81].

The triggering mechanisms behind mesothermal fluid immiscibility typically involve tectonic decompression, fluid mixing, or their interplay [82,83]. Pressure-induced immiscibility in the NaCl-H₂O-CO₂ system should theoretically exhibit the progressive enrichment of volatile components (N₂, CH₄, H₂S) within CO₂-rich fluid inclusion phases, showing positive correlations with CO₂-phase abundance [23,61,62,75,84]. However, the laser Raman data of inclusions from the Cuyu gold deposit show no such enrichment patterns (Figure 6C–E). Instead, the broad Th-salinity distributions (Figure 10) and H-O isotopic evidence collectively support meteoric water mixing as the dominant immiscibility trigger [45,83,85]. This mechanism effectively explains gold precipitation through consequent changes in fluid physicochemical parameters.

6.3. Ore Genesis and Exploration Indicationse

Based on the geological and tectonic setting of the deposit, two main genetic models have been proposed for the Cuyu gold deposit and other gold deposits in the JHGB: orogenic gold deposits [17,18,23,27,42,44,77] and mesothermal magmatic–hydrothermal lode gold deposits [20,86,87,88,89,90]. However, several pieces of evidence contradict an orogenic origin for the Cuyu gold deposit and other typical gold deposits in the JHGB. Firstly, while orogenic gold deposits typically derive their ore-forming fluids from metamorphic dehydration during orogenic processes [91,92,93,94], H-O isotopic data from the Cuyu gold deposit indicate the dominance of magmatic water in the initial ore-forming fluids. Secondly, S-Pb isotopic evidence demonstrates that the ore-forming materials were sourced from a deep magmatic reservoir associated with crust–mantle interaction rather than regional sedimentary–metamorphic sequences, suggesting that the host rocks did not serve as a source bed. Thirdly, typical orogenic gold deposits are predominantly hosted in lower greenschist-facies metasedimentary sequences, with gold mineralization typically postdating or being broadly coeval with the peak metamorphism of the host rocks [95,96]. In contrast, the Cuyu gold deposit is located in a geological setting devoid of metamorphic rock series. The gold mineralization shows spatiotemporal correlation with Mesozoic diorite porphyry intrusions (according to unpublished data: mineralization age of ~169 Ma vs. intrusive age of ~170 Ma), while the host rocks comprise Late Paleozoic granitoids emplaced at ~252 Ma. This compelling evidence demonstrates that the Cuyu gold mineralization is genetically unrelated to regional metamorphic processes. Fourthly, the Cuyu deposit is characterized by quartz-vein-dominated ore-bodies primarily controlled by NW–SE-trending ductile–brittle fault systems, a structural configuration consistent with mesothermal lode gold deposits controlled by fault networks [95,97,98,99,100]. Ore petrographic studies reveal that sulfide minerals (pyrite, chalcopyrite, and galena) constitute the predominant metallic phases, with native gold occurring as electrum. The alteration assemblage comprises K-feldspathzation, sericitization, silicification, chloritization, and carbonatization. These mineralogical and alteration characteristics align well with the diagnostic features of mesothermal gold deposits [31,90,98,101,102,103,104]. Comparative studies on the characteristics of the JHGB and typical orogenic gold deposits have concluded that these deposits should be classified as mesothermal magmatic–hydrothermal vein-type systems [14,18,32,77]. Both the Cuyu gold deposit and other deposits in the JHGB formed during the Middle Jurassic (180–160 Ma), representing coeval products of tectonic–magmatic–hydrothermal events triggered by the subduction of the Paleo-Pacific Plate beneath the Eurasian continent.

The Middle Jurassic (178–170 Ma) corresponded to a critical compressional event following the initial subduction of the Paleo-Pacific Plate. During this period, the partial melting of a metasomatized mantle wedge previously modified by subduction-related fluids generated enriched mantle-derived magmas. Intensive metasomatism promoted the enrichment of ore-forming elements and CO₂ in the magmatic systems. The subsequent underplating of these mantle-derived magmas induced the partial melting of ancient lower crustal rocks, producing abundant Au-enriched lower-crustal melts. The mixing of these crustal melts with minor mantle-derived magmas formed primary intermediate–acidic magmas. During emplacement, the fractional crystallization of magma generates calc-alkaline intermediate–acidic magmatic systems accompanied by metal-rich hydrothermal fluids containing significant gold deposits and other metallic elements [17,39]. Concurrently, structural studies have identified that NW–SE-, NE–EW-, and NS-orientated fault systems, including both newly formed fractures and reactivated pre-existing faults, serve as critical conduits for transporting these mineralized magmatic–hydrothermal materials [33,105]. Initial ore-forming fluids exsolved from differentiated magmas, characterized by moderate temperatures and low-to-moderate salinities, formed a CO₂–H₂O–NaCl system through the incorporation of minor meteoric water. These fluids migrated upward along fault zones, interacting with wall rocks to generate early-stage alteration assemblages including K-feldspathization and silicification.

Ore precipitation occurred through fluid immiscibility triggered by an influx of meteoric water, resulting in the volatilization of CO₂, H₂S, and CH₄. This process altered the physicochemical conditions of the fluids, destabilizing gold bisulfide complexes and reducing metal solubility, thereby inducing gold and sulfide deposition. Associated alteration during this main mineralization stage featured moderate-temperature, weakly acidic conditions manifested through silicification, chloritization, and sericitization. Progressive mixing with meteoric water ultimately transformed the late-stage fluids into a low-temperature, low-salinity NaCl–H₂O system. Post-immiscibility CO₂ combined with Ca²⁺ in the fluids to form late carbonate veinlets crosscutting ore-bodies and host rocks, marking the end of mineralization.

The Cuyu deposit shares remarkable similarities with other deposits in the JHGB regarding mineralization age, ore-controlling structures, fluid characteristics, and ore genesis. The discovery of the Cuyu gold deposit indicates a northwestern extension of the JHGB into Panshi City in central Jilin Province. This geological continuity provides critical theoretical guidance delineating new exploration targets for gold prospecting campaigns in this region.

7. Conclusions

(1) Cuyu gold deposit is a mesothermal magmatic–hydrothermal lode gold deposit related to diorite porphyrite veins. Its mineralization process can be divided into three stages: quartz + arsenopyrite + pyrite (stage I), quartz + sericite + pyrite + arsenopyrite + electrum + chalcopyrite + sphalerite (stage II) and quartz + carbonate ± pyrite (stage III). Stage II is the major mineralization stage.

(2) The initial ore-forming fluids came from magmatic water, which belong to a moderate-temperature and moderate- to low-salinity CO₂–H₂O–NaCl system. A subsequent influx of meteoric water drove progressive evolution of the fluid regime, initially transitioning into an immiscible NaCl–H₂O–CO₂ system. During the principal mineralization stage, the characteristics of fluids include a moderate temperature and low-medium salinity; these fluids ultimately stabilize into a low-temperature, low salinity NaCl–H₂O system in the final phase. Immiscible fluids formed a critical mechanism for the precipitation of gold and other metal sulfides.

(3) The ore-forming materials were sourced from the diorite porphyrite veins, and there is a close spatial, temporal, and genetic relationship between the diorite porphyrite and the Cuyu gold deposit.

(4) The Cuyu gold deposit is an extension of JHGB in the northwest and has similar ore genesis to that of typical gold deposits in JHGB, indicating that JHGB extends northwest to Panshi City in central Jilin Province. This provides theoretical support for regional gold exploration.